1. Technical Field of the Invention
The present invention relates to a pressure sensor in which a sensing chip for detecting pressure is provided in a resin-molded case, being covered with a protective material.
2. Related Art
Pressure sensors of this type have been known as suggested, for example, in Japanese Patent Application Laid-Open No. 2002-221462. As disclosed in this literature, a pressure sensor of this type includes a resin-molded case, a sensing chip provided in the case for detecting pressure, and a protective material for covering and protecting the sensing chip provided in the case.
Such a pressure sensor uses a soft material, such as a gel, as the protective material to protect the pressure sensor from the external environment. This type of pressure sensor is adapted to apply a measuring pressure force to the sensing chip through the protective material to carry out measurement.
This type of pressure sensor, however, produces bubbles in the protective material during a temperature or pressure cycle, whereby variation is caused in the characteristics of pressure transmission to the sensing chip through the protective material, leading to problematic variation in the sensor characteristics.
The inventors studied the mechanism of how the bubbles are produced in the protective material. The mechanism will be explained referring to FIG. 8 which shows a typical cross-sectional configuration of this type of conventional pressure sensor.
Among this type of conventional pressure sensors, the figure shows, in particular, a relative pressure-type pressure sensor, in which a rear surface of a sensing chip is sealed.
A sensing chip 20 has front and rear surfaces 20a and 20b, to which pressure forces P1 and P2 are applied, respectively, and is fixedly located in a case 10, so that the rear surface 20b faces a chip mount 11 of the case 10. The chip mount 11 of the case 10 is formed, in its one surface 11b, with a pressure introduction hole 16 for introducing the pressure force P2 to the rear surface 20b of the sensing chip 20.
The case 10 is provided with a cylindrical wall 17 projecting from the surface 11b of the chip mount 11 so as to enclose the pressure introduction hole 16 along a periphery thereof.
An internal space defined by the cylindrical wall 17 projecting from the surface 11b of the chip mount 11 constitutes a protective material injection chamber 18. The pressure introduction hole 16 and the protective material injection chamber 18 are filled with a protective material 40, such as a soft gel, that serves as a pressure transmission medium. The rear surface 20b of the sensing chip 20 is sealed with the protective material 40.
The first pressure force P1 in such a relative pressure-type pressure sensor (hereinafter referred to as a “relative pressure sensor”) is applied to the front surface 20a of the sensing chip, and the second pressure force P2 is applied to the rear surface 20b of the sensing chip 20 through the protective material 40.
The sensing chip 20 outputs an electric detection signal based on a differential pressure between the applied pressure forces P1 and P2. The detection signal is adapted to be outputted to an external unit through a bonding wire 30 and a conductor 15.
Since the case 10 of this type of pressure sensor is formed by molding a resin, small cracks, such as a micro crack K shown in FIG. 8, may be caused when the case 10 is molded. Such a crack K is likely to be caused at a corner 18a where stress particularly tends to be concentrated in the case 10.
The crack K grows, for example, with a temperature cycle. Then, a pressure cycle, for example, allows air to enter the case 10 through the crack K, which air causes bubbles, such as a bubble L shown in FIG. 8, in the protective material 40.